The invention is related to a circuit arrangement for monitoring operating conditions of a group of direct current devices, wherein the operating conditions are represented by different potentials occurring at a sampling point of each direct current device.
More specifically the invention is directed to such a circuit arrangement wherein the direct current devices to be monitored comprise subscriber line loops composed of a subscriber station, a subscriber line and a subscriber line interface circuit of a private branch exchange system (PBX).
There is provided an evaluating unit associated with a plurality of direct current devices which unit can be selectively through-connected to a sampling point of each direct current device. The evaluating unit has to sample specific voltage conditions and corresponding operating conditions of a DC device. As mentioned, a subscriber line loop can be one application for such a direct current device. In such a case, the different operating conditions correspond to open-loop or closed-loop conditions of the subscriber line loop. By monitoring these conditions, switching signals can be recognized for controlling functions and operations of the PBX. One example of such signals are dial tone signals. If each subscriber line interface circuit also employs an exchange supply fed into the circuit via supply resistors, the sampling connector may be a connecting point of one wire of the subscriber line which is connected to the supply resistor not directly coupled to the positive pole of the supply voltage.
The evaluating unit for monitoring consecutively potentials occurring at the different sampling points of the direct current devices is provided with a selector unit having inputs connected to respective sampling points. At the output side of the selector unit there is arranged a comparator for rating the through-connected potential occurring at a presently selected sampling point. The comparator is supplied by a predetermined reference voltage designating a threshold value for the operation of the comparator. Determined by the value of the threshold level two measuring ranges can be distinguished which ranges correspond to two possible operating conditions of the DC devices.
As long as at a specific sampling time the present potentials to be measured are definitely distinguished from the predetermined threshold value there will be a true output signal of the comparator which is definitely associated with one of these two ranges, i.e. a respective operating condition of the direct current device. The value of the potential to be measured, however, also may be approximately equal to the threshold value. In such a case, measures have to be taken that small changes of the value to be measured which can result from superposing noise do not effect a false state of the output signal. This means that any insignificant variation of the measured potential should not give rise to a change of the output signal.
It is known in general to provide comparators for comparing two voltages with an additional positive feedback network arranged between the comparator's output and the non-inverting input for achieving a hysteresis characteristic. However, such a directly connected feedback network cannot be used in the present design. Measurements of potentials at consecutively selected sampling points could interfere with each other, thus invalidating possibly even the evaluation of a potential which is in itself quite definitely separated from the threshold level.
It is, therefore, an object of the present invention to improve known circuit arrangements for monitoring operating conditions in such a manner that critical sampling conditions are effectively excluded.
Another object of the invention is to improve the circuit arrangement in such a manner that even in a critical case a true output signal of the comparator can be obtained which is definitely associated with one of two operating conditions.